The invention concerns a vacuum plasma spray system with a vacuum spray chamber in which a workpiece can be placed, with a device for producing a vacuum in the spray chamber and with a plasma gun located in the spray chamber, the position of which can be changed relative to the workpiece in at least one moving direction by means of a motor-driven drive mechanism and a respective control system.
A vacuum plasma spray system is known from the DE-OS No. 30 43 830.
By means of such vacuum plasma spray systems it is possible to provide workpieces with a coating which is applied in plasma spraying process. For this purpose the workpiece is placed in the spray chamber. The spray chamber is then evacuated. The powdery material is then sprayed in thin layers onto the workpiece by means of the gun.
In order to reliably coat the entire surface of the workpiece with the desired coating, the plasma gun is connected to a drive and control mechanism which allows the plasma gun to move at least in one moving direction relative to the workpiece. The number of axes required for the gun depends on the workpiece to be coated.
The system described in DE-OS No. 30 43 830 contains the essential parts required for drive of the gun, i.e. the entire drive mechanism, inside the spray chamber. The drive mechanism facilitates three moving directions of the plasma gun, namely a vertical motion for adjusting the spray distance, a translational motion in the workpiece axis (reciprocating motion of the gun manipulator) and a swivelling motion.
A mechanically complex moving system is required for these motions, which -- as mentioned above -- is inside the spray chamber. This means that several guide rods, cardan joints, telescopes, toothed racks with pinions are contained without protection in the spray chamber so that the extremely aggressive atmosphere of the spray chamber (vacuum, heat, dust) can affect the entire mechanic system. Therefore, reliability of the entire drive system is doubtful since owing to the mentioned ambient conditions within the spray chamber faults of or damage to the mechanical drives might easily occur.
A further disadvantage of this known system is the very large surface, on which dust can deposit, resulting from accommodating the entire mechanic system in the spray chamber. Moreover, dust may deposit in recesses or hollows from which it cannot be easily removed. In addition, such mechanic system has a large number of narrow gaps and hollows that are difficult to evacuate. Therefore, when evacuating the spray chamber, residual oxigen escapes from these gaps and hollows for some time which has unfavourable effects on the coating process.
Another known plasma spray system uses a robot inside the spray chamber for moving the plasma gun relative to the workpiece. From the kinematic point of view this design is suitable for coating complex workpieces. For coating, both robot and turntable are pushed into the chamber and the door is closed.
Hereby, all components inside the spray chamber are subject to extreme heat, dust and vacuum exposure. This particularly applies to the complex mechanic system of the robot with its drives and measuring systems as well as to the turntable.
This solution, too, has the serious disadvantage that a number of hollows and gaps exist in which powder desposits and which form air pockets. Both factors result in a deterioration of the inert atmosphere and hence affect the spray results. A further, serious disadvantage is the fact that it is difficult to clean the entire system (in this context it should be considered that, as a rule, 1 to 2 kgs powder or dust result during production runs in such plasma spray system, which remain in the chamber).